1. Field of the Invention
The present invention relates generally to a second-order Input Intercept Point (IIP2) calibration method of a mixer in a receiver of a wireless communication system and the mixer using the same.
2. Description of the Related Art
A down-converter for down-converting Radio Frequency (RF) signals in a receiver of a wireless communication system includes a mixer, a local oscillator, and other similar components for down-converting RF signals, and the mixer on/off controls a switch to perform frequency down-conversion.
FIG. 1 is a block diagram illustrating a configuration of a receiver including a general mixer in a wireless communication system according to the related art.
Referring to FIG. 1, a receiver 100 includes a transmission (TX) chain 110 for performing frequency up-conversion, filtering, and other similar operations on transmission signals, a power amplifier 120 for amplifying the power of the transmission signals, an antenna 130, a duplexer 140, and a reception (RX) chain 150 for performing frequency down-conversion, filtering, and other similar operations on reception signals.
The RX chain 150 includes a Low Noise Amplifier (LNA) 151 for amplifying the reception signals, a mixer 153 for performing frequency synthesis of an RF signal and a Local Oscillation (LO) signal for frequency down-conversion, a Local Oscillator (LO) 155 for generating the LO signal, and a LO buffer 157 for buffering the LO signal. The mixer 153 performs frequency down-conversion by synthesizing an RF signal of a high frequency band received through the antenna 130 with the LO signal to output a baseband signal of a low frequency band.
The gain, noise, linearity, power consumption, and other similar characteristics of the mixer 153 are important factors for determining a performance of the mixer 153. In the case of a Frequency Division Duplex (FDD) system in which a TX chain and a RX chain operate together, for example, in an FDD system of a wireless communication system, such as a High Speed Packet Access (HSPA) system and a Long Term Evolution (LTE) system, the influence of Inter-Modulation (IM) due to transmission (TX) leakage, such as TX leakage of a TX jammer flowing from the TX chain to the RX chain through a duplexer, is also considered as one of important factors for determining the performance of a mixer.
In order to remove TX leakage, such as that from the TX jammer, in the receiver 100 illustrated in FIG. 1, a Surface Acoustic Wave (SAW) filter 160 is provided between the LNA 151 and the mixer 153. However, it is not preferable that the receiver 100 includes an additional device such as the SAW filter 160. Accordingly, a receiver having a passive switching mixer, instead of a SAW filter, may be used since the passive switching mixer has relatively lower power consumption than an active switching mixer, such as a Gilbert cell mixer, and has high linearity in view of third-order nonlinearity. Also, the passive switching mixer is less influenced by flicker noise since no Direct Current (DC) flows through the passive switching mixer. However, since the second-order nonlinearity or second-order Input Intercept Point (IIP2) of the passive switching mixer are decided by mismatch factors of a frequency down-converter in a receiver, like the active switching mixer, there is still reception sensitivity deterioration due to the TX jammer.
The IIP2 is a factor indicating the linearity of a circuit. Interference between channels having a small frequency difference or interfering signals in signal bands may distort a reception signal, and IIP2 is defined to indicate the linearity of a circuit through a relationship between the amplified amount of an input frequency and the amount of distortion due to second-order Inter-Modulation (IM2) among factors causing signal distortion. In more detail, if the power of a signal received by a receiver continues to increase, then the power of an IM2 distortion signal may also sharply increase. IIP2 is an amount of power at which the reception signal is expected to intersect the IM2 distortion signal, as seen from an input terminal of the receiver. Accordingly, in order to ensure high linearity in a wireless communication system, the IIP2 should be high, which leads to minimization of IM2 distortion. Thus, in a wireless communication system, a receiver should be designed such that a parameter representing IIP2 has a great value.
Also, in a mixer, causes of IM2 being a factor for determining IIP2 may be largely classified into three components, namely a component corresponding to a size of a mixer configured with transistors or a mismatch in a threshold Voltage (VTH) according to the size of the mixer, a component corresponding to coupling of different leakage signals between an RF input and an LO input, and a component corresponding to a path of a LO signal or a mismatch of LO AC-coupling. Accordingly, in order to optimize IIP2 and/or in order to improve layout symmetry and matching, a mixer may have a large size. Also, in order to make IIP2 stable against changes in Process-Voltage-Temperature (PVT) properties without using a SAW filter, studies into various IIP2 calibration methods are conducted.
As an example of related-art IIP2 calibration methods, there is a method of unequally adjusting the output symbols of a mixer to optimize IIP2. However, the method is mainly applied to a Gilbert-type active mixer having mixer load, and switches used in tunable mixer load may increase flicker noise of the mixer or deteriorate the linearity of the mixer, resulting in IQ imbalance. As another example, there is a method of performing unbalanced DC or DC offset injection on a baseband signal path to cancel the mismatch of a mixer. However, this method may produce an additional DC offset in a baseband filter terminal.
As another IIP2 calibration method, there is a method of finely adjusting a gate bias of a switch in a mixer to calibrate a threshold voltage or a size mismatch of the mixer. This method may reduce flicker noise or a DC offset, however, this method needs an accurate resolution setting for a Digital Analog Converter (DAC) used to control the mixer since IIP2 is sensitive to the gate voltage of the switch of the mixer. Also, due to an increase in a number of bits of control signals for controlling the mixer, the configuration of the DAC is complicated so that delay is generated upon calibration of IIP2.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.